Field emission displays are desirable for use in viewfinders of video cameras and other display applications requiring low operating power and high pixel density. Such displays typically include an anode electrode coated with an cathodoluminescent phosphor, an array of field emitter tip cathode electrodes, and a perforated grid or gate electrode adjacent the emitter tips. A high positive voltage (e.g., +1,000 volts) is connected to the anode, and a lower positive voltage (e.g., +50 v.) is connected to the grid.
A pixel is the smallest independently-controllable area of the display. Each pixel includes a number of emitter tips which are controlled together. A typical field emission display has an array of tens of thousands of pixels arranged in a matrix of hundreds of rows and hundreds of columns, so that each pixel is uniquely identified by the row and column to which it belongs.
A field emission display may be "active matrix" or "passive matrix". In an active matrix field emission display, each pixel includes a control circuit connected to the emitter tips in that pixel which controls the voltage at the emitter tips and the current flow through the tips. The emitter tips and control circuit are replicated at each of the thousands of pixels in the display.
In operation, when a pixel is to be dark, that pixel's control circuit raises the voltage at that pixel's emitter tips to a value high enough that the difference between the emitter tip voltage and the grid voltage is less than the voltage required to initiate a field emission discharge from the emitter tips. Conversely, when a pixel is to emit light, the pixel's control circuit reduces the voltage at the emitter tips to a value sufficiently lower than the grid voltage (i.e., the control circuit makes the emitter tip voltage sufficiently negative relative to the grid electrode voltage) so as to cause field emission of electrons from the emitter tips. Each emitter tip emits electrons toward the grid, but almost all the electrons are accelerated by the anode voltage so as to pass through apertures in the grid and strike the phosphor coating on the anode, thereby exciting the phosphor to emit light.
An example of an active matrix field emission display is disclosed in commonly-assigned U.S. Pat. 5,357,172 to Lee et al., entitled "Current-Regulated Field Emission Cathodes for Use in a Flat Panel Display in Which Low-Voltage Row and Column Address Signals Control a Much Higher Pixel Activation Voltage".
In a raster scan video system, an analog or digital video luminance signal is produced by scanning the brightness of the video image from the left-most column to the right-most column within the top row of the image, then scanning from left to right within the next lower row, and so forth until the bottom row of the image is scanned. At this point the scanning process is repeated. At any instant, the value of the video luminance signal is proportional to the brightness of the pixel at the current scan position.
The video signal in a raster scan system includes both the luminance signal and a synchronization signal from which the current scan position can be derived. A video decoder circuit in the display receives the video signal and typically generates from it row select and column select logic signals which respectively identify, at any instant, the row and column coordinates of the current pixel, that is, the pixel whose luminance is represented by the instantaneous value of the video luminance signal.
In order to pack as many pixels as possible within a given substrate area, it is desirable to minimize the amount of circuitry which must be fabricated at the site of each pixel. Generally, the pixels are fabricated on a certain area of a substrate, and the video decoder circuit is fabricated on the substrate outside the pixel area. Consequently, electrical conductors must be fabricated on the substrate to connect each pixel to the video decoder circuit and to a source of electrical current (such as electrical ground). Many conventional designs require four conductors connecting each pixel to a binary row enable signal, a binary column enable signal, a luminance signal, and electrical ground, respectively.
Such a large number of electrical conductors connecting each pixel to the video decoder and to a source of electrical power occupies substantial space on the pixel area of the substrate, thereby limiting how closely the pixels can be spaced together. There is a need for a design enabling a display to be manufactured with a minimal number of electrical conductor lines connecting to each pixel and, preferably, a minimal number of electrical devices fabricated at each pixel site on the substrate.